1. Field of the Invention
The present invention relates generally to direction finding systems for transmitted signals.
2. Description of the Related Art
The use of photonic devices in microwave circuits has become more prevalent due to their wide bandwidths, the low attenuation losses associated with fiber-optic cables, immunity to electromagnetic interference, and the ability to implement “true time” delay lines. The revolution in microwave photonics has rendered new techniques for generating microwave (MW) signals, photonic signal processing including microwave delay line filters for beam forming applications and photonic analog-to-digital converters (ADCs). Wideband optical technology also plays a significant role in many direction finding (DF) and digital receiver architectures.
Especially important in DF applications is the use of the Mach-Zehnder modulator (MZM) built using LiNbO3 technology. In a wideband fiber-optic DF application, a MZM is used as an optical down conversion circuit at the antenna and as a means to efficiently couple the local oscillator into the optical domain. Accuracy better than 0.1 deg is achieved at 10 GHz with a bandwidth capability of 2-18 GHz.
Another photonic DF application uses optical Bragg cells. A laser is split between two Bragg cells whose outputs are Fourier transformed by a lens and detected by a channelized phase detector. An alternate approach to measuring the direction of arrival (DOA) for a radio frequency (RF) signal uses two cascaded MZMs biased at the minimum transmission point to suppress the optical carrier wave. Identical RF signals with a given phase shift are applied to the electrode of each MZM. The phase difference between the signals is determined by measuring the optical signal's power.
Another technique for detecting the DOA of broadband microwave (MW) signals in phased-array antennas is based upon a two-tap photonic transversal MW filter and measuring the frequency of the notches produced over the broadband signal using a spectrum analyzer. A recent technique utilizes a broadband, multi-emitter DF system with a dual-drive Mach-Zehnder modulator and a spatial-spectral (S2) material based spectrum analyzer that provides fine angular and spectral resolution for spectrally non-overlapping signals.